1. Field of the Invention
Activated carbons are high porosity, high surface area materials used in industry for purification and chemical recovery operations as well as environmental remediation. Toxic metal contamination of various water sources is a significant problem in many parts of the United States. Activated carbons, which can be produced from a number of precursor materials including coal, wood and agricultural wastes, are now being actively utilized for remediation of this problem. Carbon production is an expanding industry in the United States, with a present production rate of over 300 million pounds a year and a growth rate of over 5% annually. The present invention relates to the development of specifically modified carbons from low-density agricultural waste products that possess enhanced adsorption properties with regard to the uptake of metal ions.
2. Description of the Prior Art
The production of carbon, in the form of charcoal, is an age-old art. Carbon, when produced by non-oxidative pyrolysis, is a relatively inactive material possessing a surface area limited to several square meters per gram. In order to enhance its activity, a number of protocols have been developed. These include chemical treatment of the carbonaceous material with various salts or acids prior to pyrolysis, or a reaction of the already pyrolyzed product with high temperature carbon dioxide or steam. Activated carbon is able to preferentially adsorb organic compounds and non-polar materials from either liquid or gaseous media. This property has been attributed to its possession of a form which conveys the desirable physical properties of high porosity and large surface area.
Usmani et al., in their paper entitled xe2x80x9cPreparation and Liquid-Phase Characterization of Granular Activated-Carbon from Rice Huskxe2x80x9d, [Bioresource Technology, 48, (1994), pp.31-35] teach a process for the preparation of granular activated carbons from both high- and low-ash rice husks by the use of zinc chloride in the dual functions of an activating agent and a binder.
Morgan et al., in a publication entitled xe2x80x9cBinders and Base Materials for Active Carbonxe2x80x9d, [Industrial and Engineering Chemistry, Vol. 38, No. 2, (1946), pp.219-227] disclose that various glucose carbohydrates differ markedly as materials for activated carbons; with dextrose behaving as a binder, cellulose as a base material, and starch having properties intermediate to either.
Arida et al., in xe2x80x9cProduction of High Quality Adsorbent Charcoal from Phil. Woods II. Granulated Activated Carbonxe2x80x9d, [Philippine Journal of Science, Vol. 121, No.1, pp.31-52], disclose the formation of good quality granulated activated carbons from coconut coir and ipil-ipil when utilizing molasses as a binder.
In Release No. 0483.95 from the Office of Communications, United States Department of Agriculture, (1995), it is disclosed that granular activated carbons effective in the removal of metals may be created from agricultural wastes such as sugarcane bagasse as well as the ground hulls of soybean, cottonseed and rice. The process disclosed utilized black strap molasses as a binding agent and includes the steps of creating charcoal from briquettes in an oxygen-free furnace at over 480xc2x0 C. and subsequent roasting in the presence of steam at 700xc2x0 C. to create enhanced surface area.
Rivera-Utrilla et al., in a paper entitled xe2x80x9cEffect of Carbon-Oxygen and Carbon-Nitrogen Surface Complexes on the Adsorption of Cations by Activated Carbonsxe2x80x9d, [Adsorption Science and Technology, (1986), 3, pp.293-302] details adsorption studies of Na+, Cs+, Ag+, Sr2+ and Co2+ utilizing carbons prepared from almond shells that had been activated with CO2 at 850xc2x0 C. for 8 hours and oxidized with air at 300xc2x0 C. for 45 hours.
Molina-Sabio et al. in their paper entitled xe2x80x9cModification in Porous Texture and Oxygen Surface Groups of Activated Carbons by Oxidationxe2x80x9d, [Characterization of Porous Solids II, Rodriguez-Reinoso et al. (edit.), 1991, Elsevier Science Publishers B. V., Amsterdam] disclose that while oxidation treatment of fruit pits by either air or chemical means (HNO3 or H2O2) does not substantially modify the microporosity of the carbon structures created, the chemical nature of the carbon surface is changed considerably. No projected uses for these carbons are set forth.
Periasamy et al., in an article entitled xe2x80x9cProcess Development for Removal and Recovery of Cadmium from Wastewater by a Low-Cost Adsorbent: Adsorption Rates and Equilibrium Studiesxe2x80x9d, Ind. Eng. Chem. Res., 33, pp.317-320, (1994), show that at a concentration of 0.7 g/L, activated carbon produced from peanut hulls was able to achieve an almost quantitative removal of Cd(II) present at a concentration of 20 mg/L in an aqueous solution at a pH range of 3.5-9.5.
Moreno-Castilla et al., in an article entitled xe2x80x9cActivated Carbon Surface Modifications by Nitric Acid, Hydrogen Peroxide, and Ammonium Peroxydisulfate Treatmentsxe2x80x9d (Langmuir, 1995, 11, pp.4386-4392), disclose the principle that acidic oxygen surface complexes are formed on activated carbons as a result of their treatment with either gas or solution phase oxidizing agents; and that inclusion of these complexes effect changes in the behavior of activated carbons when used either as adsorbents or catalysts.
While various methodologies for the creation of activated carbons exist, there remains a need for the creation of alternate viable and cost-effective products possessing enhanced adsorption characteristics.
We have now developed a novel process, which when carried out within specific operational parameters, effects the creation of activated carbons from low-density lignocellulosic agricultural waste possessing enhanced activity for the adsorption of metal ions. This method involves activation with either carbon dioxide or steam followed by atmospheric oxidation. It has now been discovered that the utilization of a relatively low temperature atmospheric oxidation step in conjunction with carbon dioxide or steam activation of the low-density agricultural waste carbon produces metallic binding oxygen functions in the mesopore and macropore regions of the carbon. Carbons produced by this process show metal adsorption capacities greater than that possessed by existing commercial carbons.
In accordance with this discovery, it is an object of the invention to provide a means for the creation of high quality metals-adsorbing carbons.
Another object is to provide activated carbon materials having high metal-adsorbing capacity.
Other objects and advantages of the invention will become readily apparent from the ensuing description.
The present invention involves the creation of activated carbons from low-density lignocellulosic agricultural wastes, which possess enhanced adsorption ability with regard to metal cations. The carbon source for the activated carbons of the present invention may be any lignocellulosic material of plant origin having a combined cellulose and hemicellulose content greater than or equal to fifty percent (dry weight) and possessing a bulk density of less than 0.5 grams per cubic centimeter when measured for particles possessing a size range of 10 to 20 U.S. mesh. Exemplary materials include soybean hulls, rice hulls, cottonseed hulls, rice straw, wheat straw, oat straw, barley straw, sugarcane bagasse, corn cobs and peanut shells; with soybean hulls, peanut shells, rice straw and sugarcane bagasse being preferred.
According to the present invention, should a granular carbon product be desired, the low-density agricultural waste may optionally first be formed into pellets, briquettes, or extrudates by combination with a binder such as molasses, coal tar, or wood tar, before their conversion into a char. Relative ratios of the agricultural waste:binder may range from about 1:1 to about 6:1 (w/w), with a range of about 2:1 to about 4:1 being preferred. These formed precursor products are then carbonized in an inert atmosphere at temperatures ranging from about 700xc2x0 C. to about 750xc2x0 C. for a time ranging from about 1 hour to about 2 hours. The briquetted chars resulting from this process are then mechanically milled to a particle size no larger than about US 10 mesh so as to ensure their complete activation and oxidation under the conditions utilized in this invention. Chars resulting from pellets and extrudates, possessing maximal dimensions of 3 mm diameter and 10 mm length, are subjected to activation and oxidation without further size reduction. There is no effective limit to the minimum useable particle size, however, if a granular type product is desired, then it should be no smaller than about US 80 mesh.
Activation of the carbons is carried out by contact of the char material with carbon dioxide or steam under conditions and for sufficient time such that activation has been effected throughout the matrix of the particles of lignocellulosic material. The reaction is largely governed by transport phenomena involving diffusive processes. Particle size of the char material utilized affects the rate and degree of achievable activation. In order to achieve activation throughout the char material, particle size for the char should be no larger than about US 10 mesh, or possess cylindrical dimensions in excess of 3 mm diameter or 10 mm length. There is no effective limit to the minimum useable particle size, however, if a granular type product is desired, then it should be no smaller than about US 80 mesh. With this in mind, useable temperatures for the activation reaction may range from about 800xc2x0 C. to about 950xc2x0C., and for times ranging from about 3 to about 12 hours in the case of granular carbon particles in the size range of US 10 mesh to US 80 mesh. For the case of smaller carbon particles, the skilled artisan would be able to readily determine the appropriate temperature and time conditions that would effect the activation process. The charring and activation reactors may be operated under a slight positive pressure to ensure that no atmospheric air takes part in these reactions. During activation, burn-offs ranging from about 20% to about 60% are envisioned as necessary for producing the products of the instant invention. The exact level of burnoff utilized is within the purview of the skilled artisan, and is dependent upon the specific materials and reaction conditions employed. xe2x80x9cBurn-offxe2x80x9d is defined as the weight loss of carbon source, as determined on a dry weight basis, that occurs during the activation process.       Burn    ⁢          -        ⁢    off    =                              Wt          ba                -                  Wt          aa                            Wt        ba              xc3x97    100  
where:
Wtba=dry weight before activation
Wtas=dry weight after activation
Burn-off can only be between 0 and 100%. Too little burn-off (e.g., less than about 20%) is indicative of inadequate surface area and porosity development during activation. Burn-offs in excess of about 60% generally cause concomitant decreases in product surface area and functionality due to excessive pore enlargement. In addition, larger burn-offs become uneconomical due to the reduction in the amount of product produced.
The activated carbon is then oxidized by exposure to air at a temperature ranging from about 260xc2x0 C. to about 400xc2x0 C. for a time ranging from about 3 to about 6 hours. Oxidation of the carbon brings about the formation of polar functional groups on the surface of the meso- and macropores of the carbonized material. It is theorized that these are instrumental in the ability of the carbon to adsorb metal cations and anions such as those selected from the group consisting of Cu(II), Zn(II), Ni(II), Cd(II), Pb(II), Cr(III, VI), Hg(II), Fe(II, III), Au(I), Ag(I), V(IV,V), U(IV), Pu(IV), Cs(I), Sr(II), Al(III), Co(II), and Sn(II,IV).
The following examples are intended to further illustrate the invention and are not intended to limit the scope of the invention which is defined by the claims.
All percentages herein disclosed are by weight unless otherwise specified.